The rate of expansion of Universe

Transcription

1 The rate of expansion of Universe Consider a sphere of radius r=r(t) χ, If energy density inside is ρ c Total effective mass inside is M = 4 πρ r 3 /3 Consider a test mass m on this expanding sphere, For Test mass its Kin.Energy + Pot.E. = const E m (dr/dt) / G m M/r = cst (dr/dt) / - 4 πg ρ R /3 = cst cst>0, cst=0, cst<0 (dr/dt) / = 4 πg (ρ + ρ cur ) R /3 where cst is absorbed by ρ cur ~ R (-) AS 40 Cosmology 1

2 Typical solutions of expansion rate H =(dr/dt) /R =8πG (ρ cur + ρ m + ρ r + ρ v )/3 Assume domination by a component ρ ~ R -n Show Typical Solutions Are " $ R $ t # n # n = (curvature constant dominate) n = 3( matter dominate) n = 4( radiation dominate) n ~ 0(vaccum dominate) : ln( R) ~ t Argue also H = (/n) t -1 ~ t -1. Important thing is scaling! AS 40 Cosmology

3 Lec 4 Feb AS 40 Cosmology 3

4 Where are we heading? Next few lectures will cover a few chapters of Malcolm S. Longair s Galaxy Formation [Library Short Loan] Chpt 1: Introduction Chpt : Metrics, Energy density and Expansion Chpt 9-10: Thermal History AS 40 Cosmology 4

5 Thermal Schedule of Universe [chpt 9-10] At very early times, photons are typically energetic enough that they interact strongly with matter so the whole universe sits at a temperature dictated by the radiation. The energy state of matter changes as a function of its temperature and so a number of key events in the history of the universe happen according to a schedule dictated by the temperature-time relation. Crudely (1+z)~1/R ~ (T/3) ~10 9 (t/100s) (-/n) ~ 1000 (t/0.3myr) -/n, H~1/t n~4 during radiation domination T(K) Radiation Matter Neutrinos decouple pp! 6 ~ 10 s! + e e ~ 1s He D ~100s 0.3Myr Recombination z AS 40 Cosmology 5 After this Barrier photons free-stream in universe

6 A summary: Evolution of Number Densities of γ, P, e, υ All particles relativistic N N ï " R # =$ % & Rï '! 3 PP P Protons condense at kt~0.1m p c! e e Neutrinos decouple while relativistic!! + e #! + e " " A + A "! +! v v Num Density Electrons freeze-out at kt~0.1m e c e! P + H H ! R 9 10! 3 10! Rï Now AS 40 Cosmology 6

7 A busy schedule for the universe Universe crystalizes with a sophisticated schedule, much more confusing than simple expansion! Because of many bosonic/fermionic players changing balance Various phase transitions, numbers NOT conserved unless the chain of reaction is broken! p + p - <-> γ + γ (baryongenesis) e + e + <-> γ + γ, v + e <-> v + e (neutrino decouple) n < p + e - + v, p + n < D + γ (BBN) H + + e - < H + γ, γ + e <-> γ + e (recombination) Here we will try to single out some rules of thumb. We will caution where the formulae are not valid, exceptions. You are not required to reproduce many details, but might be asked for general ideas. AS 40 Cosmology 7

8 What is meant Particle-Freeze-Out? Freeze-out of equilibrium means NO LONGER in thermal equilibrium, means insulation. Freeze-out temperature means a species of particles have the SAME TEMPERATURE as radiation up to this point, then they bifurcate. Decouple = switch off = the chain is broken = Freeze-out AS 40 Cosmology 8

9 A general history of a massive particle Initially mass doesn t matter in hot universe relativistic, dense (comparable to photon number density ~ T 3 ~ R -3 ), frequent collisions with other species to be in thermal equilibrium and cools with photon bath. Photon numbers (approximately) conserved, so is the number of relativistic massive particles AS 40 Cosmology 9

10 energy distribution in the photon bath dn dh! KT c 10!9 hv = 5KT c c hv # hardest photons AS 40 Cosmology 10

11 Initially zero chemical potential (~ Chain is on, equilibrium with photon) The number density of photon or massive particles is : n = g h 3 "! 0 ( d& ' exp 4) 3 p 3 ( E / kt) % # $ ± 1 + for Fermions - for Bosons Where we count the number of particles occupied in momentum space and g is the degeneracy factor. Assuming zero cost to annihilate/decay/recreate. E = c p + ( mc )! cp relativistic cp >> mc! m c + 1 p m non relativistic cp < mc AS 40 Cosmology 11

12 As kt cools, particles go from From Ultrarelativistic limit. (kt>>mc ) particles behave as if they were massless 3 " # kt $ 4! g y dy n = % & ) => ' c ( 3 y (! h) e ± 1 0 n ~ T 3 To Non relativistic limit ( θ=mc /kt > 10, i.e., kt<< 0.1mc ) Here we can neglect the ±1 in the occupancy number mc 3 " 3 mc # 4! g # kt # y kt ( ) ~ 3 (! h) 0 n = e mkt $ e y dy => n T e AS 40 Cosmology 1

13 When does freeze-out happen? Happens when KT cools 10-0 times below mc, run out of photons to create the particles Non-relativisitic decoupling Except for neutrinos AS 40 Cosmology 13

14 particles of energy E c =hv c unbound by high energy tail of photon bath dn dh! KT c 10!9 hv = 5KT c c hv # hardest photons ~ # baryons If run short of hard photon to unbind => "Freeze-out" => KT c hv c 5 AS 40 Cosmology 14

15 Rule 1. Competition of two processes Interactions keeps equilibrium: E.g., a particle A might undergo the annihilation reaction: A + A "! +! depends on cross-section σ and speed v. & most importantly the number density n of photons ( falls as t (-6/n), Why? Hint R~t (-/n) ) What insulates: the increasing gap of space between particles due to Hubble expansion H~ t -1. Question: which process dominates at small time? Which process falls slower? AS 40 Cosmology 15

16 Rule. Survive of the weakest While in equilibrium, n A /n ph ~ exp( θ). (Heavier is rarer) When the reverse reaction rate σ A υ is slower than Hubble expansion rate H(z), the abundance ratio is frozen N A /N ph ~1/(σ A υ) /T freeze N N A ph Freeze out σ A υ LOW (v) smallest interaction, early freeze-out while relativistic σ A υ HIGH later freeze-out at lower T mc Question: why frozen while n, kt A n ph both drop as T 3 ~ R -3. ρ A ~ n ph /(σ A υ), if m ~ T freeze AS 40 Cosmology 16

17 Effects of freeze-out Number of particles change (reduce) in this phase transition, (photons increase slightly) Transparent to photons or neutrinos or some other particles This defines a last scattering surface where optical depth to future drops below unity. AS 40 Cosmology 17

18 Number density of non-relativistic particles to relativistic photons Reduction factor ~ exp(- θ), θ=mc /kt, which drop sharply with cooler temperature. Non-relativistic particles (relic) become *much rarer* by exp(-θ) as universe cools below mc /θ, θ So rare that infrequent collisions can no longer maintain coupled-equilibrium. So Decouple = switch off = the chain is broken = Freeze-out AS 40 Cosmology 18

19 After freeze-out Particle numbers become conserved again. Simple expansion. number density falls with expanding volume of universe, but Ratio to photons kept constant. AS 40 Cosmology 19